1. Field of the Invention
The present invention relates to a method of crystallizing a silicon layer to form a uniform layer of polycrystalline silicon having large grain sizes by improving the beam profile of the laser used for crystallization despite a reduced overlapping ratio.
2. Discussion of the Related Art
FIG. 1 is a graph representing grain sizes versus the energy density of an excimer laser when an amorphous silicon layer is irradiated with a single pulse from the excimer laser. Referring to FIG. 1, in an energy level region 1, although the laser energy level increases, the grain size increases slowly. In an energy level region II, the grain size increases sharply with the laser energy. When the laser energy density increases pass a level 11 into an energy level region III, the grain size decreases abruptly with the further increase of laser energy. Thus, when a laser beam is applied to an amorphous silicon layer to form polycrystalline silicon, the grain size of the polycrystalline silicon depends on the density of the laser beam to a great extent.
In the energy level region I, a portion of the amorphous silicon layer near the lower surface thereof (a lower interface) never melts since the laser energy density is low. In this case, grains start to grow as solidification proceeds from micro-crystal seeds present in the unmelted portion of the silicon layer to an upper, melted part of the silicon layer. As grains grow in a vertical direction, the grain sizes are small and depend less on the variation of the laser energy density.
In the energy level region II, most of the amorphous silicon near the lower interface is melted, but some parts of the silicon near the lower interface still remain unmelted. In this case, lateral crystal growth precedes from seeds present in the unmelted silicon portion to the melted silicon portion. Thus, the grain size increases rapidly with the laser energy density. The grain size is inversely proportional to the density of the seeds. As a result, the grain sizes formed using a laser in the energy level region II are more than ten times greater than those in the region I.
As shown in FIG. 1, the variations of the grain sizes are extremely large depending on the laser energy density, which means that the process window (the range of suitable laser energy levels) is very narrow since a small variation in laser energy density results in a large variation of the grain size. Hence, when laser crystallization proceeds in region II, it is difficult to maintain the accuracy of the equipment to accommodate the narrow process window, and mass-production is also difficult due to low yield.
In the energy level region III, there is no grain remaining at the lower interface as the amorphous silicon layer is completely melted. In this case, during the process of solidifying the melted silicon, nuclei occur and grow in many localities. Thus, minute grains are formed.
In general, as shown in FIG. 2, laser crystallization of a silicon layer is carried out by scanning a pulse laser beam 25 across a dehydrogenated amorphous silicon layer 23. The laser beam has a spacial profile of a Gaussian shape or a flat peak, and the successive laser pulses spatially overlap on the silicon layer.
FIG. 3 shows a prior art profile of a laser beam used for laser annealing of an amorphous silicon layer. Referring to FIG. 3, the laser beam profile has a flat peak region 32. When the laser beam is scanned from left to right in an overlapping manner over the silicon layer, the portion 31 of the profile to the left of the flat peak region 32 is a trailing region (or energy density decreasing region) and the portion 33 of the profile to the right of the flat peak region 32 is a leading region (or energy density increasing region). The dotted line 34 represents the laser energy level which fully melts the silicon layer.
In the above prior art method, in order to form large silicon grains of uniform sizes, the grain sizes are increased by setting the energy density level in the flat peak region 32 so that the grains at the lower interface of the silicon layer are not fully melted but the rest of the grains are melted, and by increasing the number of laser beam irradiations.
When irradiating a silicon layer, the laser beam must be irradiated repeatedly in an overlapping manner. The number of laser beam irradiations is proportional to the overlapping ratio of the laser beam. It is preferable to reduce the overlapping intervals of the laser beam to increase the sizes of the grains. The overlapping ratio according to the related art amounts to 99 to 98%.
When laser annealing is carried out using the prior art laser profile, the number of laser beam irradiations at a given point is 100 shots when the overlapping ratio is 99%, and 50 shots when the overlapping ratio is 98%. This lengthens the processing time and reduces productivity.
Moreover, the prior art uses a laser energy which leaves the lower interface of the silicon layer intact and melt the rest of the silicon layer. This energy region has a narrow process window. Accordingly, mass-production is difficult and irregular-sized grains are formed when the laser energy varies.